diff --git a/app/routes/matches.py b/app/routes/matches.py
index a9dc0cf..6c42699 100644
--- a/app/routes/matches.py
+++ b/app/routes/matches.py
@@ -48,7 +48,12 @@ def api_events():
     for tryout in tryouts:
         for match in tryout.matches:
             match_color = '#10b981' if match.match_type == 'team_vs_team' else '#f59e0b'
-            match_desc = match.description or ''
+            # 'description' used to be participants_str + '<br>' + description.
+            # Building presentation markup inside a JSON field is what carried
+            # the stored XSS: the browser dropped it straight into innerHTML,
+            # and player usernames travelled through it unescaped. The two
+            # values are already separate keys, so the concatenation also made
+            # the modal show the participants twice.
             participants_str = ''
             if match.match_type == 'team_vs_team':
                 teams = []
@@ -56,14 +61,12 @@ def api_events():
                     teams.append(match.team1.name)
                 if match.team2:
                     teams.append(match.team2.name)
-                participants_str = f"{' vs '.join(teams)}"
-                match_desc = participants_str + (f"<br>{match.description}" if match.description else '')
+                participants_str = ' vs '.join(teams)
             else:
                 player_names = []
                 for p in match.participants.all():
                     player_names.append(p.player.username if p.player else 'Unknown Player')
                 participants_str = ', '.join(player_names) if player_names else 'No players'
-                match_desc = participants_str + (f"<br>{match.description}" if match.description else '')
 
             start_time_str = match.start_time.strftime('%H:%M') if match.start_time else None
             end_time_str = match.end_time.strftime('%H:%M') if match.end_time else None
@@ -79,7 +82,7 @@ def api_events():
                 'type': 'match', 'color': match_color,
                 'extendedProps': {
                     'location': match.location or tryout.location or 'TBD',
-                    'status': match.status, 'description': match_desc,
+                    'status': match.status, 'description': match.description or '',
                     'match_type': match.match_type,
                     'tryout_id': tryout.id, 'match_id': match.id,
                     'start_time': start_time_str, 'end_time': end_time_str,
diff --git a/app/templates/pages/calendar.html b/app/templates/pages/calendar.html
index b984ad0..2290230 100644
--- a/app/templates/pages/calendar.html
+++ b/app/templates/pages/calendar.html
@@ -205,14 +205,16 @@ function fetchTryoutOptions() {
     fetch('/matches/api/manageable-tryouts')
         .then(function(r) { return r.json(); })
         .then(function(data) {
+            // new Option() sets the label as text; concatenating it into
+            // innerHTML let a tryout title carry markup into the page.
             var sel = document.getElementById('createTryoutSelect');
-            sel.innerHTML = '<option value="">-- Select a tryout --</option>';
+            sel.replaceChildren(new Option('-- Select a tryout --', ''));
             var today = new Date().toISOString().split('T')[0];
             data.forEach(function(t) {
                 // Only show tryouts that haven't ended
                 var tryoutEndDate = t.end_date || t.date;
                 if (tryoutEndDate >= today) {
-                    sel.innerHTML += '<option value="' + t.id + '">' + t.title + ' (' + t.date + ')</option>';
+                    sel.appendChild(new Option(t.title + ' (' + t.date + ')', t.id));
                 }
             });
         })
@@ -224,70 +226,124 @@ function fetchTeamOptions() {
         .then(function(r) { return r.json(); })
         .then(function(data) {
             var sel = document.getElementById('createTeamSelect');
-            sel.innerHTML = '<option value="">-- Select a team --</option>';
+            sel.replaceChildren(new Option('-- Select a team --', ''));
             data.forEach(function(t) {
-                sel.innerHTML += '<option value="' + t.id + '">' + t.name + '</option>';
+                sel.appendChild(new Option(t.name, t.id));
             });
         })
         .catch(function() {});
 }
 
+// ---------------------------------------------------------------------------
+// Safe DOM builders
+// ---------------------------------------------------------------------------
+// Everything rendered in the event modal originates from a JSON endpoint,
+// where no HTML escaping applies. Text therefore goes through textContent,
+// never through innerHTML.
+
+function makeEl(tag, className, text) {
+    var node = document.createElement(tag);
+    if (className) { node.className = className; }
+    if (text !== undefined && text !== null) { node.textContent = text; }
+    return node;
+}
+
+function detailItem(label, valueNode, fullWidth) {
+    var item = makeEl('div', 'detail-item' + (fullWidth ? ' full-width' : ''));
+    item.appendChild(makeEl('span', 'detail-label', label));
+    var value = makeEl('span', 'detail-value');
+    value.appendChild(valueNode);
+    item.appendChild(value);
+    return item;
+}
+
+// Renders newlines as <br> without letting any other markup through.
+function multilineNode(text) {
+    var fragment = document.createDocumentFragment();
+    String(text).split('\n').forEach(function(line, index) {
+        if (index > 0) { fragment.appendChild(document.createElement('br')); }
+        fragment.appendChild(document.createTextNode(line));
+    });
+    return fragment;
+}
+
+// A team block: its name, plus an optional list of player names.
+function teamNode(name, players) {
+    var team = makeEl('div', 'match-team');
+    team.appendChild(makeEl('span', 'team-name', name));
+    if (players) {
+        var list = makeEl('ul', 'team-players-list');
+        players.forEach(function(player) {
+            list.appendChild(makeEl('li', null, player));
+        });
+        team.appendChild(list);
+    }
+    return team;
+}
+
+function versusNode(left, right) {
+    var wrap = makeEl('div', 'match-teams');
+    wrap.appendChild(left);
+    wrap.appendChild(makeEl('div', 'match-vs', 'vs'));
+    wrap.appendChild(right);
+    return wrap;
+}
+
+function buildTeamsNode(props) {
+    var sides = props.participants.split(' vs ');
+
+    if (props.match_type === 'team_vs_team' && sides.length >= 2) {
+        return versusNode(teamNode(sides[0]), teamNode(sides[1]));
+    }
+
+    if (props.match_type === 'player_vs_player' && sides.length >= 2) {
+        return versusNode(
+            teamNode('Team 1', sides[0].split(', ')),
+            teamNode('Team 2', sides[1].split(', '))
+        );
+    }
+
+    return document.createTextNode(props.participants);
+}
+
 function showEventModal(event) {
     var props = event.extendedProps;
     var title = event.title;
     var type = props.type;
     var date = event.start ? event.start.toDateString() : '';
     
-    var content = '<div class="detail-grid">';
-    content += '<div class="detail-item"><span class="detail-label">Type</span><span class="detail-value">';
-    content += '<span class="badge badge-' + (props.match_type === 'team_vs_team' || props.match_type === 'player_vs_player' ? 'success' : 'warning') + '">';
-    content += (props.match_type === 'team_vs_team' ? 'Team Match' : (props.match_type === 'player_vs_player' ? 'Player Match' : 'Player Scrim')) + '</span>';
-    content += '</span></div>';
-    content += '<div class="detail-item"><span class="detail-label">Title</span><span class="detail-value">' + title + '</span></div>';
-    content += '<div class="detail-item"><span class="detail-label">Date</span><span class="detail-value">' + date + '</span></div>';
-    content += '<div class="detail-item"><span class="detail-label">Location</span><span class="detail-value">' + (props.location || 'TBD') + '</span></div>';
-    content += '<div class="detail-item"><span class="detail-label">Status</span><span class="detail-value">';
-    content += '<span class="badge badge-' + (props.status || 'scheduled') + '">' + (props.status || 'scheduled') + '</span>';
-    content += '</span></div>';
-    
-    // Add team separation for matches
+    // Built as DOM nodes rather than concatenated HTML. Every value below —
+    // match title, location, description, and above all the participant list,
+    // which is made of user-chosen usernames — comes from a JSON API and has
+    // never been HTML-escaped. Assigning it to innerHTML executed it.
+    var grid = makeEl('div', 'detail-grid');
+
+    var typeLabel = props.match_type === 'team_vs_team' ? 'Team Match'
+                  : (props.match_type === 'player_vs_player' ? 'Player Match' : 'Player Scrim');
+    var typeBadgeClass = (props.match_type === 'team_vs_team' || props.match_type === 'player_vs_player')
+                       ? 'success' : 'warning';
+
+    grid.appendChild(detailItem('Type', makeEl('span', 'badge badge-' + typeBadgeClass, typeLabel)));
+    grid.appendChild(detailItem('Title', document.createTextNode(title)));
+    grid.appendChild(detailItem('Date', document.createTextNode(date)));
+    grid.appendChild(detailItem('Location', document.createTextNode(props.location || 'TBD')));
+
+    var status = props.status || 'scheduled';
+    grid.appendChild(detailItem('Status', makeEl('span', 'badge badge-' + status, status)));
+
     if (type === 'match' && props.participants) {
-        content += '<div class="detail-item full-width"><span class="detail-label">Teams</span><span class="detail-value">';
-        if (props.match_type === 'team_vs_team') {
-            var teams = props.participants.split(' vs ');
-            if (teams.length >= 2) {
-                content += '<div class="match-teams">';
-                content += '<div class="match-team"><span class="team-name">' + teams[0] + '</span></div>';
-                content += '<div class="match-vs">vs</div>';
-                content += '<div class="match-team"><span class="team-name">' + teams[1] + '</span></div>';
-                content += '</div>';
-            } else {
-                content += props.participants;
-            }
-        } else if (props.match_type === 'player_vs_player') {
-            var parts = props.participants.split(' vs ');
-            if (parts.length >= 2) {
-                content += '<div class="match-teams">';
-                content += '<div class="match-team"><span class="team-name">Team 1</span><ul class="team-players-list"><li>' + parts[0].split(', ').join('</li><li>') + '</li></ul></div>';
-                content += '<div class="match-vs">vs</div>';
-                content += '<div class="match-team"><span class="team-name">Team 2</span><ul class="team-players-list"><li>' + parts[1].split(', ').join('</li><li>') + '</li></ul></div>';
-                content += '</div>';
-            } else {
-                content += props.participants;
-            }
-        } else {
-            content += props.participants;
-        }
-        content += '</span></div>';
+        grid.appendChild(detailItem('Teams', buildTeamsNode(props), true));
     }
-    
+
     if (props.description) {
-        content += '<div class="detail-item full-width"><span class="detail-label">Description</span><span class="detail-value">' + props.description + '</span></div>';
+        grid.appendChild(detailItem('Description', multilineNode(props.description), true));
     }
-    content += '</div>';
-    
-    document.getElementById('modalTitle').textContent = 'Match Details';
-    document.getElementById('modalContent').innerHTML = content;
+
+    var modalContent = document.getElementById('modalTitle');
+    modalContent.textContent = 'Match Details';
+    var target = document.getElementById('modalContent');
+    target.textContent = '';
+    target.appendChild(grid);
     
     // Reset buttons
     document.getElementById('deleteMatchBtn').style.display = 'none';
diff --git a/tests/test_calendar_api.py b/tests/test_calendar_api.py
new file mode 100644
index 0000000..51427f6
--- /dev/null
+++ b/tests/test_calendar_api.py
@@ -0,0 +1,145 @@
+"""Contract of the calendar JSON API.
+
+SEC-XSS-001. /matches/api/events used to build presentation markup into its
+`description` field:
+
+    match_desc = participants_str + f"<br>{match.description}"
+
+The browser dropped that value straight into innerHTML, so a player username
+— free text at the time — reached the DOM of every coach and administrator
+who opened the calendar. Escaping cannot be the browser's job alone here;
+the API must not emit markup in the first place.
+
+These tests assert the API contract. The DOM side is fixed in
+calendar.html, which now builds nodes with textContent.
+"""
+
+from datetime import date, time
+
+import pytest
+
+from app.extensions import db
+
+
+@pytest.fixture
+def match_factory(app):
+    """Create a tryout with one player_scrim match and one participant."""
+    from app.models import (
+        Match, MatchParticipant, Tryout, TryoutRegistration,
+    )
+
+    def _make(owner_id, player_id, *, description=None, username=None):
+        from app.models import User
+
+        with app.app_context():
+            if username is not None:
+                # Written straight to the column: edit_profile now rejects
+                # this, but rows predating that fix can still hold anything.
+                player = db.session.get(User, player_id)
+                player.username = username
+
+            tryout = Tryout(
+                title='Spring tryout', game='Valorant', date=date(2030, 5, 1),
+                created_by=owner_id, status='upcoming',
+            )
+            db.session.add(tryout)
+            db.session.flush()
+
+            db.session.add(TryoutRegistration(
+                tryout_id=tryout.id, player_id=player_id))
+
+            match = Match(
+                tryout_id=tryout.id, title='Scrim A', description=description,
+                date=date(2030, 5, 2), start_time=time(18, 0), end_time=time(19, 0),
+                match_type='player_scrim', created_by=owner_id,
+            )
+            db.session.add(match)
+            db.session.flush()
+            db.session.add(MatchParticipant(
+                match_id=match.id, player_id=player_id))
+            db.session.commit()
+            return tryout.id, match.id
+
+    return _make
+
+
+def _match_event(payload):
+    return next(e for e in payload if e['id'].startswith('match_'))
+
+
+class TestCalendarEventPayload:
+    def test_description_is_returned_verbatim(
+        self, app, client, as_role, make_user, match_factory
+    ):
+        player_id = make_user('player')
+        admin_id = as_role('admin')
+        match_factory(admin_id, player_id, description='Bring your own peripherals')
+
+        props = _match_event(client.get('/matches/api/events').get_json())['extendedProps']
+
+        assert props['description'] == 'Bring your own peripherals'
+
+    def test_description_does_not_carry_generated_markup(
+        self, app, client, as_role, make_user, match_factory
+    ):
+        """The <br> and the duplicated participant list are both gone."""
+        player_id = make_user('player')
+        admin_id = as_role('admin')
+        match_factory(admin_id, player_id, description='Warm-up first')
+
+        props = _match_event(client.get('/matches/api/events').get_json())['extendedProps']
+
+        assert '<br>' not in props['description']
+        assert props['participants'] not in props['description']
+
+    def test_an_empty_description_stays_empty(
+        self, app, client, as_role, make_user, match_factory
+    ):
+        player_id = make_user('player')
+        admin_id = as_role('admin')
+        match_factory(admin_id, player_id, description=None)
+
+        props = _match_event(client.get('/matches/api/events').get_json())['extendedProps']
+
+        assert props['description'] == ''
+
+    def test_participants_remain_available_separately(
+        self, app, client, as_role, make_user, match_factory
+    ):
+        """Removing the concatenation must not lose the participant list:
+        the modal still renders it, from its own field."""
+        from app.models import User
+
+        player_id = make_user('player')
+        admin_id = as_role('admin')
+        match_factory(admin_id, player_id, description='x')
+
+        with app.app_context():
+            username = db.session.get(User, player_id).username
+
+        props = _match_event(client.get('/matches/api/events').get_json())['extendedProps']
+
+        assert props['participants'] == username
+
+
+class TestLegacyHostileData:
+    """Rows written before the username policy can still hold markup."""
+
+    PAYLOAD = '<img src=x onerror=alert(1)>'
+
+    def test_a_hostile_username_is_confined_to_its_own_field(
+        self, app, client, as_role, make_user, match_factory
+    ):
+        player_id = make_user('player')
+        admin_id = as_role('admin')
+        match_factory(admin_id, player_id,
+                      description='Normal text', username=self.PAYLOAD)
+
+        props = _match_event(client.get('/matches/api/events').get_json())['extendedProps']
+
+        # The payload is data: it may legitimately appear in `participants`,
+        # which the client renders through textContent. What must never
+        # happen again is it being spliced into a field the client treats
+        # as markup.
+        assert props['description'] == 'Normal text'
+        assert self.PAYLOAD not in props['description']